Automatic Frequency Control (AFC) has become a standard feature in high-performance television receivers. As is well-known, such circuitry detects deviations from the nominal IF, 45.75 MHz, and develops an error signal in response thereto. The error signal is applied to a voltage-controlled element, e.g. varactor diode, in the receiver's local oscillator circuitry in such manner so as to reduce deviations from the nominal IF.
As elsewhere in the receiver's circuitry, standard integrated circuits are now being used to implement many AFC-related functions, thereby improving performance and reducing circuit complexity. Specifically, a Toshiba integrated circuit designated as 7611 effectively performs both the IF amplifier and the AFC detector functions. Briefly, a sample of the IF signal is coupled to the input of the 7611 AFC detector and it develops differential output signals in response to the IF. However, the output signal levels developed must necessarily be conditioned according to the characteristics of the receiver's local oscillator. The subject invention is directed to an AFC interface circuit that performs this function.